Donating money is usually something that as an individual, you look at the service provided and access what it is worth before giving an amount of money away. This goes for anything, including donations of bloggers.
I have seen bloggers putting up a donation module of some sort, whether it be Chipin, Paypal itself, or some other module that accepts payment. Considering some of the sites I have visited, I really do see a point to give some money, especially if I have learned or taken away something from the website.
It works much like web developers who offer free tools that other users online can use. This gives an extra appreciation to the developer to create more great programs for others to use or to improve upon what is already offered.
For bloggers, this could be an extra boost from readers to say they appreciate the blogger or have actually learned from the blogger something that has made their business or website successful.
For me, I think donations for bloggers are acceptable as long as the blog owner is not literally begging for money. It is one thing to put up a message to ask, but another to constantly bombard visitors with the message.
There will be loyal visitors who come and might want to support your site. Whether it is through donating up front, or advertising, sponsored posts or links, or social network sponsored post (through Twitter and Facebook), it is okay. For blog site owners, never expect it to be something common. I can attest that it is not a frequent happening.
However, it you have a few dollars (or whatever your monetary value may be named), think about supporting the blog that helps you the most in a unique manner. You never know, with the power of blogging, the thanks in return could be fruitful.
Have you donated to a blogger because you learned from them? What do you look for when supporting a blogger monetarily, other than commenting, pay for advertising, linking, social bookmarking or other means?