The venerable Xtrlock.
Transparent screenlock.

It's still ancient in how it authenticates: ought to be 
adapted to use PAM or call 'su' rather than directly 
accessing the shadow file; it's probably from the days 
when the crypted password was actually in /etc/passwd !

It compiled fine on Gentoo, but I couldn't make it work 
in Fedora 16 in a reasonable time of a few minutes: always
the problem of:
	"password entry has no pwd"
even when compiled with -DSHADOW_PWD and installed with 
permission to read the shadow file (on gentoo and my attempts,
this is done by the ugly setuid root; on debian, from which 
the soure-code was found, it's done by setgid in a shadow 
group). 

Simply copying the binary from my Gentoo system to Fedora
make it work ok.  Still curious, the commands used in the 
Gentoo build were studied, to give the two-part creation
of a working xtrlock:

gcc \
	-march=k8 -O2 -pipe -msse3 \
	-DSHADOW_PWD \
	-I/usr/include \
	-Dlinux -D__amd64__ \
	-D_POSIX_C_SOURCE=199309L -D_POSIX_SOURCE \
	-D_XOPEN_SOURCE -D_BSD_SOURCE -D_SVID_SOURCE \
	-D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -DFUNCPROTO=15 -DNARROWPROTO \
	-c -o xtrlock.o xtrlock.c

rm -fv xtrlock

gcc \
	-o xtrlock \
	-march=k8 -O2 -pipe -msse3 \
	-DSHADOW_PWD \
	-Wl,-O1 -Wl,--as-needed \
	-L/usr/lib64 \
	xtrlock.o -lcrypt -lX11

# installation
cp xtrlock /usr/bin/xtrlock
chown root:root /usr/bin/xtrlock
chmod 4711 /usr/bin/xtrlock

Working ok.  But much nicer if not setuid root.  
Must some day change that function in the xtrlock.c .