What are the best low cost ($500 and less) unlimited domains web-panels?

cPanel is a very popular option. It tends to run from $25-40 per month depending on provider and how you license it (e.g. internal or external). Annually speaking, that would seem to fit your budget.
-mike

Actually my intention was for a permanent license, which would exclude, CPanel, Plesk and Ensim.

Then I'd have to say pretty much your only option is DirectAdmin which features a Lifetime License for a one time fee of $299, which is well within your budget. Based on reviews from others, DirectAdmin is a very strong compeditor to cPanel feature wise, although it doesn't even have half of the market share. If you're on a budget, or you are looking for a lifetime license, go DirectAdmin. You can check out their pricing at http://www.directadmin.com/pricing.html
If you're not familiar with DirectAdmin features, you can check those out here
http://www.directadmin.com/features.html

I also would highly recommend directadmin. It is the absolute best, lightweight, fully-featured panel in my opinion!

I would highly recommend DirectAdmin. You could get a license for about $299 I believe.

If you have a low budget why not to look at free web Panels? I think there is some you have to look at as for Direct Admin its not the best choice yes its easy to use, light, and so on but at the same time its a panel for beginners, and its lack some useful options...

Which options do you feel that DA lacks?
I haven't found a single one! :)

Actually my intention was for a permanent license, which would exclude, CPanel, Plesk and Ensim.
DirectAdmin will be the best option for you then...

Yeah Directadmin is good option , even cpanel is also a best control panel. Cheer

Direct admin is even cheaper than $299 if you buy from a reseller i known to you it may not cost more than $100 i think so for bulk purchase only thing is you should know one person selling this at this rate,
For reseller the license charge is $14 per month for our install and $18 for DA company installation, also its less than $100 for a full life time internal license, we bought a server from Qala internet services Singapore like this.

but at the same time its a panel for beginners, and its lack some useful options...
I highly disagree. I find DirectAdmin allows for much better and intuitive control for both clients and administrators than cPanel. What 'useful options' in your opinion is it missing? I did a large cPanel -> DirectAdmin migration a while ago and never had a customer complain of missing features.
For the OP requirements I would also recommend testing out DirectAdmin.

I highly disagree. I find DirectAdmin allows for much better and intuitive control for both clients and administrators than cPanel. What 'useful options' in your opinion is it missing? I did a large cPanel -> DirectAdmin migration a while ago and never had a customer complain of missing features.
For the OP requirements I would also recommend testing out DirectAdmin.
I agree with you and i am sure Direct admin is having all features that cpanel have and runs on lower memory and resources in the server, allow more options, simple in look.
May be it will take some time for users to shift from Cpanel to DA and this is in the hands of DA company they are not promoting it aggressively

I would recommend DirectAdmin as well. See also http://www.site-helper.com/

Again, I've run into the same issue. Most of my clients who have switched from cPanel servers have never looked back. DA can do everything that cpanel can do with a more intuitive, more responsive, and with less of a memory footprint than can the behemoth cPanel. Also, DA is isolated somewhat from the server itself whereas cPanel is fully integrated into it. That means if something breaks it takes cPanel with it or vice versa. You're restricted to what cpanel supports. With DA, you can add what you need and because they are all somewhat isolated, just because one fails, it doesn't take everything with it.
That's one of the reasons cPanel is such a memory hog (though much better now that they have optimized it), because it has so many requirements and dependencies, that it has become bloat software for many! :)

For all this the OP should give a try for DA server then he can understand

They offer a demo on their site as well.

Demo and also support is really great if you mail to support@directadmin.com surely you can get a reply within some 24 hours this is my personal experience with DA

I highly thank everyone for their responses. I have done research before and heard of DirectAdmin, but have only used CPanel. I would like to graduate from a dedicated server to colocation, and therfore am doing this research. I have a local server on my desktop using Linux. I wonder if I can test run it, before purchasing?

Demo and also support is really great if you mail to support@directadmin.com surely you can get a reply within some 24 hours this is my personal experience with DA
OK, I will try emailing them this week, thankks!