Nowadays it's possible to kit our your new computer without spending a penny on software. The programs listed below are either open-source, made by non-profit organisations or (in one case) are part of a company's 'freemium' business model. All of them, without exception, are top of their league, being far better than practically all paid-for programs made by big tech firms.
Honourable Mention: Mozilla Firefox
The only reason we haven't put it on the list is because you probably already have it - it is a very well-known alternative web browser, and indeed for a while was the best one out there, hands-down. While this writer thinks that's still the case, some people have moved to Chrome for its light weight and ease-of-use. Still, Firefox is a true browser pioneer: it was the first browser to have Tabs, and to have a Private Browsing mode. Indeed, it was the first one in which entering a search term in the URL field would generate a Google search, as opposed to returning an error. Without it, Chrome wouldn't exist and IE wouldn't have had the impetus to get its act together.
A nifty little tool that cleans all the gunk out of your system. All the temporary files, Recycle Bin files, temporary internet files, caches, and so on... After running it, you will notice a performance boost. Run it a few times a week, and it will keep your machine sprightly and nippy. One of computing's great satisfactions is running it on a long-suffering machine owned by a tech-unsavvy relative: you can almost hear the thing breathe a sigh of relief.
The open source community's response to Microsoft Office. OpenOffice Writer (the MS Word equivalent) is an incredibly strong viable alternative, being lightweight, fully-functional and able to edit MS Word documents (and many other file types). While it may not have the sleek graphics of Office 2010, it makes up for it in speed and, quite frankly, in being free. A great piece of kit.
Audacity is the best sound recording software you can find, apart from professional industry packages (and even then it holds its own with some!). The fact that it's open-source is probably its strength: it's constantly being updated with new features, effects and plugins - making it a formidable tool that outclasses, outguns and outmatches all other free, consumer and pro-sumer packages, paid or free. Want to record your band, multi-channel, on a variety of inputs, all at the same time and remaster each track? Want to slow your favourite song down by 3000% and preserve the pitch? Or perhaps you want to make a voice recording and do anything from a basic sweetening to making your sound like Darth Vader? Audacity can do it, and more.
Another open source piece of kit, the VideoLan Media Player (to give it its proper name) is the best media player out there, far more versatile and powerful than Windows Media Player, Quicktime or any other corporate offerings. It comes with a huge array of codecs installed, meaning it can play virtually any music or video file you throw at it. It also has powerful functionality beyond that of a player - it can re-export files as different formats, rip video streams off the Web (ie from YouTube), record video from webcams and much more.
MalwareBytes Anti Malware
While MBAM doesn't give you real-time protection (unless you pay for the upgrade, that is), it does give you access to a database of bleeding-edge virus profiles (automatically downloaded each time you run the program), meaning it's the best anti-virus scanner out there. Think you've got a bug in your system? Update the database, run a full scan and MBAM will find it. Whereas others (I'm looking at you, Norton) are cumbersome and lack up-to-date virus knowledge, this one really knows what it's doing.