It depends on how your website/web application is used, but browser caching can give you fantastic performance increases for very little effort. Especially with a database driven site that is primarily read-only (many ecommerce sites), proper browser caching can decrease the number of pages you serve per user, which in turn increases the number of users supportable by a given set of hardware.
I found this caching tutorial to be very helpful in understanding just how to cache pages, as well as RFC 2616, which states in section 13.2.4 that the Cache-control: max-age header takes precedence over the Expires header (for the browsers who speak HTTP 1.1). This examination of the support of various browsers is also excellent reading. There’s also a cool tool called the Cacheability Engine which examines caching behavior of web pages, if you don’t want to look at the headers yourself (using Fiddler or LiveHTTPHeaders). I encountered occasional errors with the engine, but it was pretty neat to use.