You don't want 'substitutes for software', you need 'solutions'
Apparently you want to: manage files burn and rip cd's make websites without looking at html code browse the web with a decent set of plugins.
You can manage your files with konqueror. It will let you view your files (I guess that's what you use irfan for), look inside archived files (winzip), and transfer files from/to ftpsites (cuteftp).
Rip your audiocd's with konqueror. Burn your cd's with karson or gcombust.
Make websites with OpenOffice (don't worry, unlike msoffice, openoffice produces nice, clean code)
Gimp is excellent for making webgraphics. It also features a script-fu collection. If you're not any good at making webbuttons, let gimp create them for you...
Flash may run using wine/win4lin/... But I generally advise against using flash. Why? Because flash5 isn't an open, w3c accepted standard. Blind users will have problems navigating your site. PDA/cellphones/third-party programs will have problems too. If you feel up to it, I would suggest going for an XML solution. (see the tutorial at ibm for an introduction of why it would be better).
You can use Galeon or Mozilla to browse the web. Both feature tabbed browsing (an absolute must if you do any serious browsing), integrated search abilities, and galeon also features a download manager. (mozilla features it in 1.1alpha), or use an external one.
Get a basically the same set of plugins on linux you have on windows using codeweavers crossover. Quicktime. (flash and realplayer have native versions).
If you think of 'replacing your software', you'll end up a wintroll. If you think of 'doing something', you'll end up being a happy user.