There are two good, that is, relatively inoffensive, ways to cuss online:
The first and simpler is to use a bunch of hyphens, perhaps with an exclamation point, for instance:
"What did you think of that --------- Obama's speech last night?"
"What did I think? -----------------!!"
The second method is to hold down the shift key and run your thumb over the numbers from two to eight; an exclamation point can be added for emphasis, for instance:
"That @#$%^&* car I bought is nothing but a @#$%^&* lemon!!"
The beauty of this is that it leaves the actual word or words up to the reader's imagination, which will come up with better (worse?) words than you could. Plus your readers can't accuse you of being offensive or dirty-minded, because they're filling in the blanks, not you.