According to some threads on the Nexus the site is having some problems and a lot of people are experiencing them. Hopefully they'll get them fixed soon. I'm sure we'll hear about it from Dark0ne if it's anything more than minor hiccups.
Same problem here. Very annoying, 'cause I was just downloading loads and loads of mods for my fresh Oblivion reinstall.. Let's hope it's up again in a few moments/hours.
I can't access any the NMM website or there forum , i was just using there forum 20 minutes ago now that isn't responding either whatever is going on seems to be widespread and involves the NMM and it's servers i hope your right that it's a hiccup but it doesn't sound like it to me.
Update: I found threads mentioning clearing cache works. I did that and I manage to download mods from some servers (manual download). Not all servers, though, so try a few servers before giving up.