Domain Registration Is Cheap

You can get a webhost like Blue Host web Hosting which will allow you to host unlimited domains with just one account that costs just $6.95/month. So once you've got an account like this then your only cost is the actual Domain Registration fee which is less than $10 per year per Registration at Go Daddy. Often times there are deals on certain types of domains (such as .info) where you can get a Domain for as cheap as $2/year!

Domains Gain Value With Age

It's been shown that the search engines respect older domains more than new domains. If you have an idea, buy the Domain now and put up a simple site for it. Even a single page that says "coming soon" is better than nothing. If you link to it a couple of times then you can get it indexed (this is important) so that it can start gaining "age" in the eyes of the search engines. Again with the low price of Domain Registration there's no point in waiting. Even if you don't get to working on it for a year, you'll be glad you already bought it and it's already got past part of the "aging" process.

Parking With Go Daddy

A cool feature that Go Daddy offers is that they will "park" your Domain with Google ads on them for a small fee. You can then actually make money from those Google ads which often times will actually pay for the Domain Registration anyway and will help you get the pages indexed without having to go through and make page for each of them.

Going Too Far

You may get a bit addicted with buying domains up. I don't recommend going too far with it unless perhaps you are trying to "farm domains" but that's a whole other subject and it's not something I do so I can't tell you about it. In my view you can go too far with buying up domains every time you have an idea. If you find yourself with far more domains in the "someday" stash then in the "currently working" stash then you may have gone too far with your Domain buying.