FOXIT READER is a light weight PDF reader, only around 5 MB, and yet it provide more feature then Adobe Reader. I just tried it out the other day and I love it, can totally replace my Adobe Reader.
Once you download and installed the application you will notice the install process is lighting fast. Moreover, the speed open any PDF document is much faster than Adobe Reader. This is evident when you have a large PDF documents.
An experiment you can do it yourself. Adobe Reader opening a 35 MB e-book took around 20-30 seconds to complete. With Foxit Reader it opened instantly.
Some of the additional function it provide that I found very useful:
The ability to highlight text
The ability to add comments
The ability to view text only (pure text with no format)
Before with format
after without format
The only minor problem I ran into with this software is that it opens up the new documents in a new tab, so you can’t preview all your open PDF documents like you do with Adobe Reader in Windows 7/Vista mouse hover.
But regardless everything else, simply for normal reading, the speed it brings is enough to convince me to switch to this as my default PDF reader. 
Try it out yourself, but keep in mind that the free version will leave evaluation water mark when you making additional comments etc.
You can download Foxit Reader here
Update: as one of our twitter follower points out you can go to the Preference (Ctrl + K) and enable "Allow Multiple Instances" and uncheck "Tabbed documents" to make it behave exactly like Adobe PDF reader with the preview thumbnail in the taskbar.
