Bitcoin has been used for unsolicited tipping since before it built up any value.
This page aims to give people some advice for tipping bitcoin online.
Usually it is considered impolite to send unsolicited private messages on IRC. As a result, it is usually tolerated to ask someone for a tipping bitcoin address in the public channel, which they may opt to answer to publicly or in private at their choice.
Third party services
ChangeTip is a great service that allows integrated bitcoin tipping on many sites:
- Reddit: http://www.reddit.com/r/changetip/wiki/index
Misconstruing as spam
Bitcoin tipping can sometimes be misconstrued as spam. To avoid this, it's best to be aware of your environment. For example, when tipping on certain Reddit thread, it is best to contribute a logical comment along with the tip.
Remember, many people don't "get" bitcoin. There's a chance many people never will, in which case bitcoin becomes more of a backend protocol. Forcing bitcoin into people's faces isn't productive for anyone.