Upload unmanaged package on AppExchange, without being Salesforce ISV Partner?

You can join the partner program as an individual ISV partner and list free apps on the app exchange (managed or unmanaged) at no cost (I've done so!). However, you'll need to be fairly persistent with folks at SFDC to keep your stuff moving, they seem to forget about the small players sometimes.

You can upload and distribute unmanaged (and even managed) packages, but you won't be able to list it "publicly." Users can still install it if they can get a link to it, for example, from your website. If you want to actually have it be searchable directly in the AppExchange, you need to get certified (i.e. pass security review), which requires being in an agreement as an ISV/OEM. Being certified makes your package "limitless", meaning the code, objects, fields, and so on don't count against the global org limits (e.g. number of objects, fields per object, number of code characters...). I have personally uploaded packages that were never certified and could be installed by any org, although not in a number of years. Some consulting companies will develop code in a managed/unmanaged package, and then provide links to their clients to install the package.