First of all, they have a Polish Dictionary. Now this is a great resource, but, personally, I don't use it. It's not that it isn't useful, and you might find that it can be your go-to resource. But I'm so tied in to the Anki flash card program that I simply use the deck browser on my Polish flash card deck as my dictionary (I've talked about Anki here and here a little bit but eventually I'll have to have a more detailed description of it in a separate post because it is definitely my favorite flash card resource for multiple languages).
The Polish Dictionary at University of Pittsburgh is only searchable, as far as I can tell...you can't actually browse it. I may be wrong about that as I've only taken a cursory look at it. If you know otherwise, feel free to comment. But there are many more useful features on this site.
First of all, there are a couple of very good reference grammar ebooks on there for free. The most accessible one is Polish Grammar In A Nutshell. This is a short summary of some of the most frequently used points of grammar, and is the one that I run to most often to answer quick questions without a huge amount of depth. But there is also a very comprehensive grammar that is nearly five hundred pages long at this link: A Grammar Of Contemporary Polish.
OK, those are the reference materials that are available on the site. There are also a series of lessons. Lessons 1-6 also appear to be contained in Volume One. There are a couple of other volumes referenced in Volume One but I haven't found them yet. There appears to be a .pdf dictionary mentioned so it may be on the site somewhere.
There are also a lot of audio files on this site. I used to come here for some audio files that were in print and audio, but I can't seem to find them any more, and the links to them on the site map don't seem to be currently active. I'm sure that what they have on there currently is really great.
Also, there are some computer drills that indicate that you have to utilize some program to use. I'm not sure if you have to install it or if it is on the web. I haven't really used them or explored through them, but they look very useful.
Keep in mind that this whole site appears to be a work in progress, and changes around. Things have moved around a lot since I first started going here, so some of the links may change. You might want to poke around the site some to see how it is organized, but I'm sure you will find some fantastic resources here for learning Polish.
But what I like about this site the most is that it appears to just totally be a labor of giving. There is no commercial component to it at all. No ads, no entreaties to buy anything, just rock-solid, useful information. This is classic 1995 Internet, when we thought the Web would be a big open-source sharing resource, before the bulk of the whole thing became a series of exploitative advertisements/data mining device/malware delivery system/porn site. See, you can still find the love out there somewhere.
Feel free to comment if you want to add more info about the University of Pittsburgh's site, talk about anything I've left out here, or if things move around and you want to update info. Or just to ruminate, blather, or say hi.