Ask Mr. Robot - This site has grown in popularity over the years and has improved greatly since it first launched. It's an alright place to start if you're an absolute beginner, but I find that that the reforging presets are often limited and/or are straight up incorrect. Sure, you can adjust some of the presets, but I personally find it rather cumbersome to do so. The site by default also has a habit of landing you under your hit/expertise caps. While I understand their rational behind doing this, as a dual-wielding class that already sees far too many parries for my liking in both soloing and in raiding situations, the last thing I want to do is undershoot my caps.
WoW Reforge - This was my personal favorite reforging resource for the longest time. Far more accurate than AMR and even ReforgeLite, WoW Reforge was an excellent and rather easy to use site. Unfortunately it doesn't seem to support the item upgrade system, hence the use of past tense. If you're not using your Valor Points to upgrade gear, this site can still be of great use. As I said, it's a pretty easy site to use overall, allowing you to to easily adjust the stat weights to your liking and while it doesn't automatically account for hit/expertise racial bonuses, those numbers are very easily adjusted and adding in multiple caps it's a snap. With everything this site has to offer, I'm very bummed that it doesn't support the upgrade system, to say the least.
ReforgeLite - My current go-to source for reforging. Unlike the previous two links, this addon allows you to make reforging changes right in game and it's fairly accurate to boot. This addon automatically takes into consideration hit/expertise racials and even automatically determines the hit/expertise required for your level (super handy for those Herald of the Titans twinks!). The biggest downside to this addon would be the fact that it is limited to two caps only. So if you're say, a Windwalker Monk who wants to hit your hit/expertise caps first, then set a haste threshold and have everything else dumped into crit or mastery once you've hit that threshold. You unfortunately won't be able to that with ReforgeLite. You'll be restricted to just getting your hit/expertise caps and will need to manually adjust for how much haste/other stats you'd like. Outside of that downfall though, this addon provides a very quick and easy way to reforge any character. Presets are readily available, and while they may not always optimal for your current gearing scheme (they are usually incorrect for Enhancement), they are ridiculously easy to readjust for your needs.
ReforgeLite - The Quick and Dirty Guide
(Click on the images to enlarge)
When you have ReforgeLite installed, you'll see the an extra window pop up whenever you visit your local Reforging Agent (shown on the right). At first it may look scary and confusing, but fret not! It's actually rather easy to use.
The red circle indicates where the preset drop down menu button is located. Clicking on that button will pop down said menu and show you all of the available presets. Click on the one that most resembles what you're aiming for (in this case, Enhancement) and the addon will automatically give you presets for hit/expertise caps as well as a secondary statistic priority order.
If the secondary stat order isn't to your liking, as it often isn't for me, you may use the little up and down arrow buttons that are located to the left of the stats to adjust their priority. Click on these arrows to adjust as needed.
Here's how I view stat priority for Enhancement: Hit/Expertise cap (7.5% for each respectively), always first and foremost.
Level 80 (Twink): Mastery > Haste > Crit
Leveling 85-90: Mastery > Haste = Crit
Fresh 90/Heroic 5 Geared: Crit (until ~20%) > Mastery = Haste
Tier 14 Geared: Mastery = Haste > Crit (Assuming ~20%)
Tier 15 Geared: Haste > Mastery > Crit Haste overtakes Mastery once you gain 2 RPPM trinkets + your legendary meta gem, actual change over threshold before these items may vary depending on gear; haste and mastery are pretty equal without RPPM items.
Tier 16 Geared: Mastery > Haste > Crit Due to the large amount of cleave/AoE encounters in Siege of Orgrimmar and Mastery's high burst potential, Mastery is often favored. Haste is king for single target situations when one has a certain gear set up (including the Assurance of Consequence trinket) and when specced into Primal Elementalist, however reforging on a per-fight basis often becomes too problematic and costly for such a small difference between the Mastery and Haste-heavy builds.
Challenge Modes: Mastery > Haste > Crit (I have 20% crit after reforges when scaled down; depending on your gear, you may need to tweak some things)
PvP: Hit/Expertise to 3% | Mastery > Haste > Crit (As suggested by Vanguards over on AJ)
Soloing Burst: Mastery > Haste > Crit Depending on situation. With certain set ups Haste may be greater than Mastery, though I personally prefer to stick to high mastery builds.
Soloing Survivability (Echo Healing): Mastery > Haste > Crit
Keep in mind that roughly 20% crit is needed for a decent Flurry uptime, which is what makes haste worth its salt for us. Without proper Flurry uptime, haste is pretty much poop. Once you hit that 20% threshold, mastery and haste over take crit, though crit is still a pretty decent stat!
Once you've settled on your reforging priority, you'll want to hit the 'Calculate' button, as indicated in the red circle in the image above. Keep in mind that on certain machines this may cause your screen/game to freeze momentarily (it does on mine). Give it a few seconds while it crunches all of your numbers and everything should be fine relatively quickly.
When the addon is done calculating, you can scroll down a bit and see which stats will be traded for what. As you can see in the image above, I'll be taking on a slight loss in mastery with this reforge, but I'll gain crit, haste, and expertise in the process.
If you are satisfied with what you see, you'll want to hit 'Show' next, again indicated in the red circle in the image above. This will pop up another box, which is overlapped on my reforge box and my character sheet in this image (it can be scooted around to where ever for your liking). This Output box will show you which items will be reforged (indicated by the lack of a check mark) and how each piece will be reforged. If you do not want an item to be reforged, say if it's trading mastery for haste and you don't want it to, simply toss a check mark in the box and it will skip over making that change. This window will also give you an idea as to how much your reforge will cost, as indicated at the bottom of the box next to the Reforge button.
To get things moving, simply hit the 'Reforge' button! The addon will automatically start feeding the items that are to be reforged through the reforger. That's right, this part of the process is all automated!
After a few seconds, the addon is finished with its job. As you can see on my character sheet, my alt shaman is now at her proper hit and expertise caps while having excess stats filtered off into haste.
When I first started using ReforgeLite I was super skeptical of it, as I tend to be with most things. If you find this addon to be confusing at first glance, you're not alone. I too found it overwhelming the first couple of times I tried to use it. However, over the past year or so I've absolutely fallen in love with ReforgeLite. While it may not necessarily always give you the most absolutely perfect, bleeding edge, completely optimum reforge for your current gear set up (you'd need to run simulations for that sort of a thing), it'll definitely put you solidly in the ballpark of good enough.
Install, enjoy, and I hope this post has helped you out with your reforging endeavors!