What do you think of web sites in which people beg for money or gifts? Sometimes they are providing a service (such as blogging), sometimes they are getting married or having a baby, sometimes they are facing serious illness or financial catastrophe – but in all cases, they throw up a web site and ask for contributions.
For a blogger to ask for donations as compensation for the time and effort he puts into providing a quality site is okay. It’s somewhat like a musician on a street corner with a box at his feet for the free-will offerings given in appreciation for his music. Donors to blogs should use ordinary prudence in determining whether or not to give to a particular blogger, but there is nothing wrong with a blogger asking for “tips.”
In all other cases you mention, my personal opinion is that virtual begging shows extremely poor taste and little class, particularly when done purely for personal gain (e.g., begging for wedding gifts on one’s “wedding web site”). Even in cases where the virtual beggar is not out for personal profit but is facing genuine hardship, it seems to me that the time and effort expended on setting up and maintaining a donations web site would be put to better use seeking assistance from legitimate non-profit foundations and/or social service agencies. In any case, I cannot recommend giving to anyone who solicits over the Internet in this way.