You can use a third party sites such as MetaGamerScore.com. These type of sites periodically scan your achievements, giving you a good overview of what achievements you have unlocked (or yet to unlock).
There is also a site named TrueSteamAchievements that offers achievement tracking (truesteamachievements.com).
I'm not sure about MetaGamerScore, but with TrueSteamAchievements the site shows you the percentage of registered users that have completed the achievement. This will give you a guide to how rare an achievement actually is. Although not always true due to other factors, it allows you to figure out the "harder" achievements to unlock. There are also tools for filtering down achievements to, say, achievements that are acquired in single player, online etc.
When registered to TrueSteamAchievements, you can add achievements to your todo list, and view solutions to each achievement (good to check criteria, but also to see if someone has found a glitch or shortcut to cut down the hours you spend on one achievement). There are a whole bunch of other functionality in the site too (and you can link it to your Xbox and PlayStation, if you own such a console).