Some Software Manager for Mac OS X like Synaptic in Ubuntu

Is there some software manager like Synpatic in Ubunut for Mac OS X.... Right now, I am using kind of old Mac OS X 10.5.8 and want to install softare on it... Like Opera, Firefox... I don't mind that the versions of browsers I can install on this Mac OS X system are not secure anymore... I will not use it that much... I can still use Safari... The problem is I don't know which versions of Opera and Firefox I can install... For example, I installed the newest Opera and now it says : > You cannot use this version of Opera with this version of Mac OS X.