If you are looking for a dedicated research tool, there
http://www.ideamason.com/ is one to check out. It's intended for serious academic input. It's a pig to work with because of its very complicated system. It also misses some of the important extra features of UR. It also constrains you to enter citations and references correctly and formally, which can be a drag if not required. Otherwise look at
http://www.zotero.org/ for formal research but you will not get the features of UR
I've actually tried all of these apps and just about all the others available. It's a tedious process and what I have learned is that you really need to define exactly what you want to achieve, or at last a framework, or else you get lost in choices and decissions.
Ultra Recall gives a very balanced feature set with flexible options to leverage personal requirements. Other apps just can't do that.