Wikipedia are currently having their annual fundraising drive and consequently a large ‘encouragement’ to donate appears on every single encyclopedia article, which is an annoyance.
Whilst it is possible to ‘collapse’ the advertisement to a much smaller one, having to do so is about as big an annoyance as the original advertisement.
A popular Wikimedia-themed newsletter, Wikizine, pointed out that by modifying the theme, it is possible to remove it.
All registered users can edit two files which modify CSS and JavaScript on Wikipedia for them. By default, the monobook theme is used and to my knowledge, this will only under that theme.
To remove the advert, ensure you are logged in, then go to Special:mypage/monobook.css. Go to ‘edit this page’ and paste the following:
/* Remove fundraiser banner and sitenotice */
#siteNotice, #fundraiser, .fundraiser-box { display:none; !important; }
Save the page and then do a hard refresh (Ctrl+F5). The advert should be gone!
Wikimedia obviously requires money in order to operate and they massively value donations, but I find begging for money in such an obtrustive fashion very annoying and it really makes Wikipedia less easy to use.

Make Wikipedia more usable with Vector
Become a Wikipedia poweruser
IndyWiki a desktop Wikipedia tool
Visual Wikipedia
WikiTaxi Takes Wikipedia Offline
I don’t see these ads.
Hey!
You shouldn’t say that Wikipedia is begging. Imagine a world without Wikipedia!
Sure the ads are obtrusive but still they are 1000 times better than those flash ads other sites have. A better option according to me would be that Wikipedia start ads. These ads should be 1 line and embedded at the bottom.
@iampriteshdesai: I have donated before and donated hundreds of hours to the project, so I find it rude they beg for more.