I have an old PowerBook 1400cs and want to put another OS on it.
I was thinking BeOS, NextStep or Linux.
How can I make this happen?

BeOS won't run on laptops, Linux is quite possible using some very old versions like Debian 2.

How about Mac OS 7.7?

Actually, these will run upto OS 9.1. Back before mine died on me it was running OS 8.6. I believe GAT here wants to try something different however.

Here is a link that could help you get Debian up and running on this, I'll probably be trying this on my Power Computing with it's 400 MHz G3 upgrade card to see how hard an old-world installation is.

It doesn't say that Debian supported the 1400, I checked NetBSD, No luck.
Would Debian work on a clamshell?

Debian will run on a Clamsell, I did so myself, on a G3\466 with 560 megs of ram.

Here is a list of OS's that have native support for the PowerPC according
to Wikipedia's PowerPC page. Note many on this list probably don't
support your particular Powerbook config (need more RAM than
you model's 64M Max, etc) but this may give you an idea of where
to look. You also may want to try a bootable Ubuntu CD, it may
not work but it doesn't hurt to try as it worked on my G3 iBook.


Apple's Macintosh System 7.1.2 through Mac OS X 10.5.8.
Linux
ArchLinuxPPC, with 32-bit releases (optionally with a 64 bit kernel)[7]
CRUX PPC, with 32 and 64 bit releases[8]
Debian, with 32-bit powerpc a released port,[9] and ppc64 in development.[10] Due to the considerations above the developers recommend using the 32-bit "powerpc" port on 64-bit systems (with an appropriate 64-bit kernel).
Fedora with 32 and 64 bit ppc releases [11] up to version 12.
Gentoo Linux, with 32-bit ppc releases[12] and 64-bit ppc64 releases. Due to the considerations above, the developers recommend using the 32-bit "ppc" release on 64-bit systems (with an appropriate 64-bit kernel).
MintPPC, Support for Old World and New World 32 and 64 bits Macs based on Linux Mint LXDE and Debian[13]
MkLinux, Mach-kernel based distribution for older Macs.
OpenSUSE, Full support for Old World and New World PowerMacs(32 & 64bit), PS3/Cell, and IBM POWER systems.
Red Hat Enterprise Linux
Slackintosh
T2 SDE, Full support for both, 32 & 64bit, Apple and IBM PowerPC systems and the PS3/Cell.
Ubuntu, Community Supported for versions released after 6.10 [14]
Yellow Dog Linux, Full support for both, 32- & 64-bit. PS3 supported.
NetBSD, port designations for PowerPC systems
ofppc released[15]
macppc released[16]
evbppc released[17]
pmppc released[18]
mvmeppc released[19]
bebox experimental[20]
amigappc very experimental[21]
FreeBSD, 32-bit powerpc released port[22]
OpenBSD, 32-bit macppc released port[23]
Xross Media Bar (PlayStation 3 Operating System)
Windows NT 3.51 and 4.0 also supported PowerPC processors
ReactOS is also being ported to the Power ISA.
AmigaOS 4
MorphOS
Plan 9
IBM AIX
IBM i5/OS
Solaris 2.5.1 PowerPC edition on the PReP platform
OpenSolaris, experimental[24][25]
BeOS R5 Pro (BeBox, Macintosh and clones)
Haiku, experimental[26]

I think your best bet is using a newer OS X system. 10.2 or 10.3 should do the trick and they will run a modern web browser and modern apps you can use to actually be productive.

Personally, i would leave MAC OS on it *uses personal opinion*

Maximum MacOS for PB1400 is the 9.1 (using a G3 upgrade and 64MB RAM).

I am running 8.5.1 on my PB1400(Sonnet G3), as the 9.1 is a bit resource hungry (even on 64MB RAM)