Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
As its name suggests, Aaron Swartz's GPL-licensed rss2email utility converts RSS subscriptions into email messages and sends them to whatever address you specify. Despite the name, it handles Atom feeds as well, so you should be able to use it with just about any feed you like.
Why would you want to receive feed updates in your inbox rather than checking them in a feed reader? Isn't the whole point of feed subscriptions to browse them at your leisure? For the most part, I don't want to receive an email every time one of the feeds I subscribe to is updated -- I have more than 200 subscriptions, so that would fill up my inbox pretty quickly. However, there are a few select feeds I do want to monitor more closely, so I use rss2email to shoot me an email when those are updated.
If you don't subscribe to many feeds, then rss2email might be a better fit for you than a full-blown RSS client. You can receive updates in the comfort of your favorite mail reader without having to set up a feed reader.