So you've got this slick little Rocksmith 1/4" to USB audio adapter, and your guitar plugs into your PC like a proverbial boss. What do you do once Rocksmith has honed your chops to a glistening, diamond-plated sheen? You wield your new rockskills to write your own blistering riffs and record some of your own music, that's what.
What do you need beside a guitar and the slick little Rocksmith audio interface? Doesn't it cost a bunch of money to rock? No, motherf*ckers, this is the 21st Century, and we have a little thing called the Internet that can provide almost everything you need to melt faces for little more than the cost of your ISP bill.
If all you want to do is plug in and jam, there are some great free options:
Native Instruments Guitar Rig Player
I've been using NI's Guitar Rig software as my main amp simulator since about 2005. It's great for everything from rock to more atmospheric and effects-drenched fare. Like the full retail version, the free version of the software works as both a standalone amp simulator and a plugin for your DAW (Digital Audio Workstation) of choice. It's got a limited selection of presets and minimal ability to edit, but it's still quite usable for rocking, and is a good taste of what the full version offers should you decide to upgrade.
IK's Amplitube is a similar product to Guitar Rig, and the free version of the software offers similar functionality. However, IK offers a "Custom Shop" where you can purchase amps and presets a la carte. You like Amplitube Free, but want more capacity for unleashing some metal fury? Swing over to the custom shop and purchase that kick-ass Engl Powerball model for $20. I wasn't much of a fan of Amplitube for the first two versions, but I think IK has really done an awesome job with Amplitube 3. Its well worth a look.
Surprisingly awesome freeware amp sim. It's a bit rough around the edges in terms of UI, but it rules for high gain stuff, and it's totally free. Works as a standalone amp sim or a VST plugin for compatible DAWs.
There are lots of other great free plugins out there, but the majority of them require DAW software to host them. You will also need a DAW if you are interested in recording, so without further ado:
Reaper, the mother of all free DAWs.
Well, it's not exactly free, but you can use it for free with no restrictions until you decide to pay the $60 fee for a personal license. There's a bit of a learning curve, but don't let the price tag fool you, this is a legit DAW and more than adequate for laying waste to the neighborhood with lethal salvos of rock. In fact, it's what Harmonix has been using for authoring tracks in Rock Band, so if you get really awesome and decide that you'd like to submit your stuff to Rock Band Network, you'll be all set.
Once you've settled into your DAW, you have access to some excellent freeware plugs: amp sims, cabinet impulse responses, effects and so on. There is a dizzying array of surprisingly high quality stuff out there, and I'll put a list together later, when I have some more time. In the meantime, I'd love to hear about any awesome free/cheap guitar-related software y'all might recommend.