A mixed bag of gardneing tools for a garden that might exist only in the user's mind
I am a big fan of Onyx which is free and stable and safe - mileage may vary on the last two but I haven't personally had any problems or read of many instances where people have had problems. It is one of those Swiss army penknife tools of useful features. Aside from freeing up a lot of space on my small hard drive on a laptop, the laptop zipped along with that like new feeling. Placebo effect? Auto suggestion? It genuinely seemed a little faster. If you have good grasp of basic terminal commands you can get almost everything it does done without the app. It is free, though.
I then developed that dreaded MS Windows obsession of 'how can I tweak my laptop to make it run faster?' On the whole I would say that is a waste of time and to just go back to using your Mac as a productivity tool; tweaking and removing parasites from Windows is like inadvertently owning an allotment that you periodically have to weed and tend to keep it running nicely. I am generalizing, there, I know but it was hard to escape that feeling, especially during the Vista years.
Aside from Onyx, Xslimmer made a difference with applications opening faster - there seem to be a lot of tools out there for stripping out unwanted languages and unneeded PowerPC code but Xslimmer apparently is one of the better ones for keeping its database up-to-date with what is and isn't safe to remove. I first used it years ago and the worst that happened was that I had to re-install an application that it slimmed. That hasn't happened to me since. I previously used the free Monolingual, carefully I thought - and had to reinstall Leopard.
Of the applications for fully un-installing applications I have a grudging respect for CleanApp, though you need to use it carefully. The reason I tried it out was that I was having problems removing MacKeeper! CleanApp logs any additions to your computer that are installed by new software, in theory allowing a 100% clean uninstall which, with big applications, dragging that application to the trash can leaves a lot behind, though usually harmlessly so; I figured if I got Cleanapp to monitor a fresh (re)install of MacKeeper I could then use CleanApp to fully remove it. It worked. Along with pointing out what new things had been installed and modified, Cleanapp also listed suggestions from the community users database of other things I might like to try to remove. There is a risk element there but I was desperate to rid myself of MacKeeper. I don't want to say anything bad about MacKeeper, it seemed like a competent suite of tools but I found I wasn't the only person who had trouble removing it. Some people on their forums were calling it a virus, which it isn't. I liked it's simple and fast duplicate file finder for removing duplicate photographs. More on that below.
CleanMyMac - I couldn't tell you exactly why but it makes me nervous. It didn't find as many fragments to remove from applications that I wanted to remove as did Cleanapp or Appzapper.
AppZapper - again, not freeware I'm afraid but I found its detection of things to remove second only to Cleanapp. It is billed as the uninstaller that Apple forgot to bundle with Snow Leopard. Perhaps all application removing apps use that for their byline, though! Up until I found Cleanapp this was firmly my favorite application remover and in many ways still is. It seems less of a gamble than (the community suggestions part; unlike Cleanapp AppZapper appears not to reference a database of people's uninstall experiences) Cleanapp. I think it would be easy to get carried away with some of the Cleanapp's community's suggestions for complete removal.
AppCleaner - I was excited to try this one as I had heard people refer to it as the poor man's AppZapper. I found it found the least number files to remove when uninstalling an application. That is not to say that is necessarily a bad thing but I am AppZapper fan which I found thorough and safe. Cleanapp I would carefully use for removing huge sprawling Adobe and Microsoft trials - and for removing MacKeeper!
Finally, and useful to photographers, removing duplicates! There are some iPhoto specific applications for removing duplicates that I could talk about but cutting to the chase the best overall duplicate remover that I found was TidyUp. It was easy to use and clear to me what I was deleting, what was and was not a close or exact duplicate, and from where - but once the limited trial timed out I didn't feel that I wanted to buy it. If I wasn't on a budget I would call it a must have, though. It was then that I moved onto the MacKeeper trial. Its duplicate finder was simple, quick and efficient, as far as I could see, though lacking in many preference search options. Some of its features are duplicated by Onyx, though, and it is debatable whether or not you need the services of it's anti virus plugin.