I'm currently matching donations for people who would not otherwise have donated, to Givewell, SENS, and SI. Please see this thread http://lesswrong.com/r/discussion/lw/8z8/holiday_giving_thread/ - there are $200 unmatched of my originally offered $1000. The $800 that is already matched will be going to SI.
On the subject of whether to give now or later, http://blog.givewell.org/2011/12/20/give-now-or-give-later/
Many LW readers choose to direct their charitable donations to SingInst with a view toward reducing existential risk. Others do not, whether because they feel they lack an understanding of the relevant issues, because they value present day humans more than future humans or because they have concern as to the incentive effects that would be created by donating to SingInst at present. I personally feel that there's a strong case for saving money to donate later on account of better information being available in the future.
However, I feel cognitive dissonance attached to saving to donate later rather than now. If you are in this camp you might consider donating to GiveWell's top ranked charities. Also note that spreading the word about GiveWell promotes a culture of effective philanthropy which is likely to have spin off effect of interesting people in x-risk reduction, reducing x-risk.
See Holden's article on last minute donations http://blog.givewell.org/2011/12/30/last-minute-donations/ :