Since I see project managers spending most of their non-meeting time in Word, Excel en Powerpoint, adding the "web-based" requirement means Google Doc or Office 365 are the right tools for them. The choice might depend on their mobile equipment and - of course - company standards.
Google Doc is free, Office 365 isn't.
What more would you need?
If some needed software isn't available on an iPad while a tablet is needed for the circumstances a laptop doesn't work (do such circumstances exist?), adding an Android or Windows tablet to the package might be a cheap and simple solution (say $200 a year, equivalent to a mere 2 hours for a free-lance project manager). BYOD is a luxury not all projects can afford.
Thanks for taking a look.
I'm looking for web-based project management platform that is simple and inexpensive (ideally free for small # of users) and has a sister iOS application.