If you are always net connected, have a look at chromium or Google Chrome.
Otherwise, try the Linux netbook flavours, they're free. OS X is also fast and nice to work with, and the price of Snow Leopard is really good. But beware of the system update 10.6.3, it doesn't work well with NetbookInstaller.
Some netbook linuxes may need some ripening -- eg. the window of disk utility in Ubuntu Netbook Remix doesn't scroll and some window areas are not accessible. Just a matter of months till they fix it, I guess.