There are two types of plugins included in the distribution:
Sooner or later all the contrib plugins should be hosted at the official 3rd Party Nagios plugin page Nagios Exchange
Contrib plugins have to be installed manually. They are not affected by the build process.
Here is a list of the actively maintained Plugins:
check_apt, check_breeze, check_by_ssh, check_clamd, check_cluster, check_dhcp, check_dig, check_disk, check_disk_smb, check_dns, check_dummy, check_file_age, check_flexlm, check_fping, check_ftp, check_game, check_hpjd, check_http, check_icmp, check_ide_smart, check_ifoperstatus, check_ifstatus, check_imap, check_ircd, check_jabber, check_ldap, check_load, check_log, check_mailq, check_mrtg, check_mrtgtraf, check_mssql, check_mysql, check_mysql_query, check_nagios, check_netdns, check_nntp, check_nntps, check_nt, check_ntp_peer, check_ntp_time, check_nwstat, check_oracle, check_overcr, check_pgsql, check_ping, check_pop, check_procs, check_radius, check_real, check_rpc, check_sensors, check_simap, check_smtp, check_snmp, check_spop, check_ssh, check_ssmtp, check_swap, check_tcp, check_time, check_udp, check_ups, check_users, check_wave
And these plugins are currently distributed in the contrib directory:
check_adptraid, check_apache, check_apc_ups, check_appletalk, check_arping, check_asterisk, check_axis, check_backup, check_bgpstate, check_breeze, check_cluster, check_cluster2, check_compaq_insight, check_cpqarray, check_digitemp, check_dlswcircuit, check_dns_random, check_email_loop, check_fan_cpq_present, check_fan_fsc_present, check_flexlm, check_frontpage, check_hltherm, check_hprsc, check_http-with-client-certificate, check_hw, check_ica_master_browser, check_ica_metaframe_pub_apps, check_ica_program_neigbourhood, check_inodes, check_inodes-freebsd, check_ipxping, check_javaproc, check_joy, check_linux_raid, check_lmmon, check_log2, check_lotus, check_maxchannels, check_maxwanstate, check_mem, check_ms_spooler, check_mssql, check_nagios, check_nagios_db, check_nagios_db_pg, check_netapp, check_nmap, check_oracle_instance, check_oracle_tbs, check_ora_table_space, check_pcpmetric, check_pfstate, check_qmailq, check_rbl, check_remote_nagios_status, check_rrd_data, check_sap, check_smart, check_smb, check_snmp_disk_monitor, check_snmp_printer, check_snmp_process_monitor, check_snmp_procs, check_sockets, check_temp_cpq, check_temp_fsc, check_timeout, check_traceroute, check_traceroute-pure_perl, check_uptime, check_vcs, check_wave, check_wins