App that scans local network for shared folders?
Finder by default will show shared folders for most services.

For an more in depth look at what a machine is running you can always port scan the heck out of it. Then connecting is as easy as memorizing proper instructions over telnet or plugging into Finders 'Connect to Server' dialog.

nmap (really sudo nmap) is one of the fastest, most informative port scanning tools out there. It spanks the pants off any GUI for speed and service comprehension. Try something like the the following.

# [target ip] = Ip of the host to scan
sudo nmap -PN -n -sV --version-all [target ip]