The other day I found out about MySQL's INSERT ... ON DUPLICATE KEY UPDATE statement. It allows you do do an INSERT when a given key doesn't exist, or to do an UPDATE statement when it does.
I have a lot of code that does a query to see if the given ID exists, and then does an INSERT or UPDATE depending on the condition. This statement should simplify such code nicely. Good thing I know about it now.
I'm not sure if this is MySQL specific or general SQL. I'm pretty firmly locked into MySQL already though, so it doesn't really matter to me.
For the Sillysoft Newsletter I wanted an email mailing list that also had an RSS feed of itself. This way users could subscribe whichever way they want. I didn't find anything that did what I wanted so I created my own using PHP and a MySQL database.
I mentioned it on the macsb mailing list offering to share my code. A few people seemed interested, so I cleaned it up a bunch and hereby release it as RSSMailingList!
If you want a combination RSS feed generator and mailing list manager then check it out.