There are thousands of plug-ins for WordPress. Many seem to do the same things. Here are a few that I have found useful and also some tips for selecting plug-ins from the repository.
Tips for Plug-ins
Every plug-in in the repository has the following pieces of information, check these:
Compatible up to:
Unless you are running an old version of WordPress for some reason, you shouldn’t have to worry about the first one, but you do want to confirm that it is compatible with the version you are using, that it has been updated recently and that the number of downloads is fairly high. I can’t give you absolutes for what “recent” and “fairly high” mean because it will depend on how useful the plug-in is, but if there have only been a five downloads and it was last updated in 2009, I would stay away from it.
On the other hand if you see something like this, you can feel confident that it is well tested and you can be comfortable installing it (this is WP Super Cache, http://wordpress.org/extend/plugins/wp-super-cache/)
Requires: 3.0 or higher
Compatible up to: 3.3.1
Last Updated: 2012-1-30
The other things to check are version number, a low number (like 0.1) implies it hasn’t been upgraded often, and the stats tab, which will show you downloads per day and can confirm the plug-in is still popular.
Some Other Popular Plug-ins
W3 Total Cache (Free)
Helps if you have migrated from a different website structure