This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
I'm guessing you are running a windows machine, and would like to have a look at Linux. If so then you could consider installing CywinCygwin. Of course you must realise that this is just an Linux emulator.
Might be just as easy to create a dual boot machine, and install a proper disto :-) - I would suggest Fedora (did like SuSE, but not now).
If you really wish to look at a UNIX operating system, then I know Sun Solaris 10 can downloaded (I think it's now open source).
I don't think you going to get onto any UNIX servers via the Internet, too many (operating system) hackers.
I don't know about free Linux server access, but I'm pretty sure if you look just a little, there's quite a number of ISPs who offer shell accounts and even entire systems for quite reasonable rates.
Of course, the quickest way is Knoppix, since you don't have to trash your existing system. Another alternative is Cygwin, which provides many Linux apps from within Windows. I run The Gimp and more than a few software development tools that way. However, cygwin isn't without its problems. Not all apps have been made cygwin-friendly and some have issues with the different ways that file paths are coded (and file access rights are implemented) between the 2 OS's.
If you just want to get comfortable with the Linux way of doing things, the above are generally OK. If you want to play around with the low-level OS stuff or use specialized hardware under Linux, then it's time to look at eithe a dual boot or a dedicated Linux box.
An IDE is no substitute for an Intelligent Developer.