RegexLib.com
Over the past 3 or 4 months I've been involved in adding some new features to RegexLib.com. Firstly, we changed the look-and-feel of the site (thanks to Thomas 'Aylar' Johansen) to give it a friendly look and to change the navigation structure.
Secondly, we implemented categories of patterns. This makes it much easier to find the type of pattern that you are looking for. Clicking on the 'Patterns' link on the top navigation bar brings you to a page where you can get a "tabbed" view of patterns by category. It's useful because, if you're looking for a pattern to match an e-mail address then you click on the "Email" tab and search through the (currently) 14 patterns in that category.
Finally, I added the ability to leave comments against patterns. So, clicking on the first pattern in the [Email] patterns category you can see that that particular pattern [ http://www.regexlib.com/REDetails.aspx?regexp_id=26 ] has generated a great deal of conversation.
So, if you're looking for a regex, I highly suggest that you "scoot" over to regexLib and take a look through the current patterns. If you find a pattern that suits your needs, leave a comment to that effect and, likewise, if a pattern is incorrect leave a message as to what would break it.
You can also Register to add your own patterns - http://www.regexlib.com/Register.aspx. In the very near future I will add the ability to filter patterns by author. This will have the effect that you can easily use RegexLib as your own, personal library of patterns.
Feel free to leave a comment if there's any other features that you'd like to see added!