Let me summarize some of the tools computers/Internet has to offer you to increase your vocab. Definitely you should look other ways as well, but IT is damn powerful and easiest way!
I would suggest you to use web extensively to learn English. These are my few suggestions:
1. Install Fire fox (Internet Browser), install a plug in called Dictionary tool-tip. When you restart your fire fox, Tool-tip will work :-) This will immensely help you to know meanings of words you encounted newly. Just double clicking on any word will pop-up a definition, you can chose any major dictionaries to do this task.
2) Read lot. Especially from the good sources, I suggest you two major sources: BBC English learning page and The New Yorker magazine (Sources provided). BBC have H2G2 community, where you can put a writeup for evaluation. H2G2 is great I tell you!
3) Install Real player. Stream Radio stations, especially NPR and BBC. This might not help you to immensely increase vocab, but definitely your English. If you are using mac, install iTunes, and stream Old time Mystery radio. If you are using Linux (like me, Ubuntu fan ;-) ) install RhythmBox or XMMS.
4) As many already pointed out, Flash cards will be of great use. Go to any book stall, get yourself a set of flash cards (Usually comes in 2000s) and start cramming. If you cant afford it, you can make beautiful Flash cards using FX (look Source) and print it out, cut it neatly.
5) Read a lot! If you are using windows OS, Install Microsoft Reader (Free download from Microsoft). You can make a good digital library of classics. Books need to be in .lit formats. I suggest you to download utorrent, and search in ISO Torrents for .lit books. When ever you see a new word, search in www.dictionary.com and make a flash card. There are many free offline dictionaries (to download) available in internet as well.
6)In yahoo games, play a game called LITERATI. That will help you great, I tell you!
7) Yahoo chat have many rooms in Education section, chat with a native English speaker will definitely help you as well.
8) If you have apple's iPod, there are many podcasts available to help your vocab. My Suggestion is: Away With Words (now known as: On Words) of NPR (I love it). You don't have to have iPod to listen podcasts anyway.
9) Subscribe to Anu Garg's Word of the day, email. A new email will wait for you each day, with a fresh (probably a new word for you, but not a "common" word). There are many word of the day mails like webster, dictionary.com etc but I find wordsmith's the best.
You can add this 'word of the day' in your home page (Google IG via gadgets or Yahoo home page via RSS). I use google/ig by the way ;-)
All the best,