https://blog.torproject.org/blog/openssl-bug-cve-2014-0160
Do not use Tor right now. The gaping hole in OpenSSL affects Tor too. Wait a few weeks, then download a new version.
See the link above.
If you run a hidden service, turn it off if you can, since it can leak information.
THIS IS IMPORTANT.
edit:
A good note by klahaya: if you have Tails version of Tor, it's OK, since it uses an old OpenSSL version.
However, I wouldn't bet on all the hidden services being safe.
New browser bundle to fix this already released.
https://blog.torproject.org/blog/tor-browser-354-released
Still, be cautious as the hidden service you use can leak information.