Chrome: If you're a heavy commenter on other web sites, it can get hard keeping track of it all. Chrome extension Comment Save logs each comment you make around the web so you can check up on them.
While a lot of sites have good comment systems that track your comments and notify you of any replies (ahem), many sites don't — especially those that allow you to comment anonymously. There is, of course, nothing wrong with commenting anonymously, except sometimes it's hard to keep track of all your comments around the net and view replies from other people. Comment Saver for Chrome logs each comment you make, stores the log locally (so it doesn't invade your privacy), and lets you view it any time you want. You can either view your most recent comment from the extension button, or view them all on one page, seeing on what site the comment was made, what the comment was, and at what time. You can also, of course, go back and view the original comment from the extension's page, too.
Note that it doesn't work on every site out there — only ones using simple text boxes (so some services, like Disqus, are not supported) — but it does work on most everything I've tried. I'm also having a bit of trouble using it on Lifehacker at the moment — it's only logging one comment at a time.
Comment Save is a free download for Google Chrome.