2010 festivals - I've hunted down sites for you, hope this is helpful! Muse appears to be the elephant in the room headlining wise, so if you're not a fan, you may want to check first to see if the festival you were planning on has them top billed.
SXSW - March - U.S. (Austin) => no-go this year b/c I passed on the opportunity from PW
Ultra Music Festival - 26-27 March - U.S. (Miami) - I'm waiting for more acts to be announced. a single woman going by herself isn't really prudent, I don't think :(
Coachella - 16-18 April - U.S. (SoCal) => no-go unless I get a press pass b/c the line-up didn't impress me
Dot to Dot - 29 May (Bristol), 30 May (Nottingham), 31 May (Manchester) - England - I really enjoyed Notts last year!
Pinkpop - 28-30 May - Holland (Landgraaf/Heerlen)
Isle of Skye - May - Scotland - hmm, site not up anymore
Ibiza Rocks - Tuesdays throughout May and September - not sure about this one b/c they were Ibiza and San Antonio last year
Rock am Ring - early June - Germany - generally heavier rock, but Moz performed there years ago
Download UK - mid June (?) - England (Donnington) - generally heavier rock
Isle of Wight - mid June - England
Glasto - June - England
Rock Werchter - July - Belgium (Leuven)
Roskilde - July - Denmark (via Copenhagen)
Oxegen - July - Ireland (County Kildare via Dublin)
T in the Park - July - Scotland (Balado, Perth and Kinross)
Latitude - July - England (Suffolk) - this one would be awesome if certain bands played it this year...
02 Wireless - July - England (London)
Benicassim - July - Spain
Download - 3 certain weekends July through August - U.S. (SF, LA, Philly?)
Cambridge Folk Festival - July into August? - England
Camp Bestival - 30 and 31 July and 1 August - this looked interesting to me b/c Rob da Bank got Friendly Fires to close out the festival Sunday night. but v. pricey.
Virgin Mobile - August - U.S. (Toronto, B'more)??
Lollapalooza - August - U.S. (Chicago)
Summer Sundae - August - England (Leicester)
Pukkelpop - August - Belgium
Frequency - August - Austria
Lowlands - August - Holland - a plus: has tents
V - August - England (Chelmsford/Staffordshire)
Reading/Leeds - August - England
Electric Picnic - August - Ireland (County Laois)
Get Loaded in the Park - August - England (London/Clapham)
Creamfields - August - I'm not entirely sure where this is. according to their "By Air" page, you should go to either Manchester or Liverpool Intl, which is entirely unhelpful. and throwing out "Merseyside" (which I know is near Liverpool) and "Warrington" (Stuart Maconie, help!) in their "By Car" directions don't help much either! :)
Isle of Wight Bestival - early September - England