In my access log, normally entries have a request for "GET/ [some page]" but occasionally I see "HEAD/ [some page]".

What does this mean? Is someone accessing my site from something other than a browser?

A HEAD request fetches just the server information string. The most common occurrance is probably coming from http://www.Netcraft.com - they attempt to survey the internet and provide statistics on web server/OS market share, etc. Lots of interesting stuff in their reports.