I too think you should check www.eurotrip.com which caters mainly for young american visiting Europe on the cheap. You should find many useful advices on their message boards.
You could want to check the europe board on www.fodors.com, too. Posters on this board are more traditionnal tourists (I mean who stay in hotels, don’t travel cheaply, etc…), but the board is very active, and you would find tons of suggestions about places to visit, trains, etc…
Also the rec.travel.europe board on the usenet, which is quite similar (there’s also an usenet board about backpacking, but less active, IIRC).
The thorntree of www.lonelyplanet.com isn’t very useful for Europe IMO because many questions get few answers (and amongst these answers quite often some abuses). It’s a much more interesting board for less travelled places (I can’t think of another travel board as useful for uncommon destinations).
I personnally wouldn’t advise to use www.virtualtourist.com, contrarily to the previous poster. I found there are a lot of errors on the personnal pages (since contrarily to a board, nobody is correcting innacurate statements), and the message board isn’t really active and the responses are often lacking in accuracy, IMO.
The only advantages of this site are that it’s fun to check the personnal pages and that there’s a lot of great pictures from various places. I wouldn’t rely on it for advices, though.
I would want to second Fretful’s response. Don’t plan to visit too much. You’ll be tired, spend all your time moving/packing/searching for a place to stay, etc…and won’t enjoy anything. Don’t be too ambitious.
Also, I wanted to correct Dmark’s post : you CAN’T buy a train ticket from Paris to Rome, hop off in, say, Lyon, spend two days there and resume your train travel. Once you begin your train travel, you’ve a limited time to finish it (staying two hours in Lyon in the example above would be OK. Two days is a no-no. How could you prove you’re not actually using for the second time an old Paris-Rome ticket, if it was true?). Anyway the price difference between two tickets for the two legs of your travel and only one Paris-Rome ticket would be extremely minimal.
And by the way, since I’m talking about train tickets, if you intend to travel by train, buy your tickets directly from the european railways companies, not from…eurorail.com or raileurope.com?..I never remember which one…anyway they add huge fees to the tickets they sell (30-40% more or so).
Also, the cheapest way to travel is by bus but they are slower/more uncomfortable/run much less often/only exist between some major towns
As for cheap places in Paris, there are :
-very cheap family-run hotels (20-30 USD/night) but you won’t find them on the net (they’re not the kind of hotels which have websites) and they are a bet…they could be awful places. Or very fine for the price. The only way to find them is to roam in the popular district of Paris (XIX°-XX° “arrondissements”, for instance) and check the hotels you see. Rather time-consuming.
-Cheap chain hotels (around 30 USD/night). Like the “Formule 1” hotels, for instance. Clean and standardized, but not very pleasant of course. They’re usually situated just outside Paris administrative limits. Check those hotels situated in a place called “Porte de whatever” , which means there are very close to Paris proper and there’s a metro station nearby. You should be able to find their sites.
-Hostels (around 20 USD/night). No privacy, but young fellow travellers, hence company and more fun. You should find some opinions about Paris hostels on the eurotrip site, though the same hostels tend to be always recommanded. Usually the most partying hostels (not necessarily the best choice for everybody). The best hostels in Paris are probably the three rarely mentionned MIJE hostels, in the fourth “arrondissement”. Centrally located, luxurious (by hostel standarts), in historical buildings, but…of course, there are some BUTs…there’s a curfew, a lot of school groups tend to stay there (not necessarily fun) and they tend to be fully reserved long in advance.